Depends on what kind of "safe" you're asking about. The blade guard for the
12" blade won't adequately protect from inadvertent contact with a 10"
blade. When the saw is pulled down for a cut, is the blade guard
sufficiently retracted?
As long as the arbor sizes are the same, I'd guess that both blades would
cut in pretty much the same way and probably cut without any difference in
safety, but using the 10" blade would negate some of the safety features
that would be used by the 12" blade.
I don't own a miter saw and even I can see some potential problems in using
a smaller blade. It's not like a table saw where most of a blade, whatever
size, is beneath the table out of harm's way.

the
Yes it will. The guard protects against everything within it. It's not so
different from using your table saw with the blade only partially up through
the table.

Not true. The only safety feature on a cutoff saw is the blade guard and
the 10" diameter fits well within this.

using
Yes it is. The blade is completely within the guard in the same manner that
a 12" would be. Not owning a miter saw makes it difficult to answer the
question posed by the OP. Erring on the side of safety is one thing but it
can reach a point of diminished returns.
Of course, most of this is pretty moot since others have pointed out that a
10 and a 12 use different arbor sizes.

Well, within limits, the smaller diameter blades will cut wood.
There are at least 3 areas of concern:
A. Upward pressure against the fence. The position of the fence with a
full 12" blade will put pressure somewhat squarely onto the face of
the fence. With the smaller blade, the work being cut may have a
tendency to quickly ride UP the fence. It's the same problem which a
radial arm saw has, only the blade cannot slide away from the fence
like it does on a RAS or SCMS.
Hazard: piece being cut explodes up off the fence in pieces
B. The blade guard may leave some unguarded blade or it may contact
the base surface of the saw at an angle with undesirable results,
depending on how the guard rises as the blade descends.
Potential hazard: guard is pushed into the blade
C. Limitation of width of cut - may leave uncut material between blade
and fence. Pushing blade deeper to enlarge may cause blade to contact
metal at bottom of blade slot.
On Fri, 17 Sep 2004 23:31:51 -0500, "The Pistoleer"

Having used smaller blades (mostly metal cutoff blades, although some wood
blades as well), I can attest that the wood will not ride up the fence. I
suppose you can reach a point where it will, but within reason it won't
since the head still pivots from and through the same points and axis.

The blade guard leaves about the same amount of unguarded blade. Remember,
the 10 is just a slightly smaller version of the 12 and it fits within the
same profile as the 12. The blade guard always contacts the stock or the
base, regardless of what's installed in it. It's designed to do exactly
that. The guard cannot be pushed into the blade since it pivots around
essentially the same point as the blade rotates.

Yup. You would certainly lose width of cut. There would be absolutely no
risk of contacting metal at the bottom of the cut though. If a 12" doesn't
contact metal, how would a 10"? The OP would certainly have to put blocking
under the stock to achieve the full width of cut though, to make up for the
smaller diameter blade.

Almost certainly not. As a start, the arbor size is different...the 12" hole is
1" in diameter, while the 10" is 5/8" (in the US). Then there's the meeting of
blade and fence, where the angle is wrong with the 10" blade, assuming you can
find a 1" to 5/8" adapter.
Save the 10" blades for table saw use.
Charlie Self
"Half of the American people have never read a newspaper. Half never voted for
President. One hopes it is the same half." Gore Vidal

can
Charlie - I'd like to use yours as the post that I sort of.... challenge, I
guess. Nothing to do with you Charlie, it's just that your voice sometimes
has a certain authority around here.
I've never put a 10 inch blade on in place of a 12, and for the sake of
discussion, let's leave the arbor size issue off to the side right now. I
have however, used cut off blades in miter saws in the past, and quite a bit
at that. These blades wear down with use and you can very quickly find
yourself going from a 10" blade to a 6" blade. Other than eventually losing
width of cut capability as the blade gets smaller I've never encountered any
other issues with the smaller blade.
Other posters have suggested the stock may rise up the fence, and I've found
that not to be the case. The angle of the saw head continues to be down
into the stock even as the blade wears down.
Others have also expressed thoughts about the blade guard not properly
protecting the blade. I know this will draw gasps of horror from some, but
I've used a miter without the guard before, so it really wasn't much of an
issue if the blade didn't profile to the guard. On other saws I've had the
blade guard in place and it presented no problems. The blade guard pivots
around the same axis, roughly, as the blade and everything within it is
covered, so a 10" blade would be covered just the same as a 12" would.
Ok, so this really wasn't much of a challenge to what you said. It was more
of a post that expresses some experiences in contrast to some of the other
posts that expressed conjecture in the name of safety. Not that I'd
recommend some of the past practices I've employed - I'm not particularly
fond of using my chop saws without the guard. For some reason that does
make me more nervous than using my table saw without a guard. But...
sometimes the speculations that we come up with just don't hold any water
even if they are in the name of safety.

Fair enough, but...I don't know the level of expertise of the OP, but it didn't
sound as if he was a long term woodworker. Recommendations that might
compromise safety aren't advisable anyway, but are even less advisable in such
cases.
Whether or not there's a big safety problem, I don't know. I have had minor
problems with shrinking blades in miter saws. I'd as soon not have any more...I
don't use frangible cut-off blades in my miter saws, but I do sometimes use
them in cut-off saws, and my personal limits have that blade out of there when
it has lost about 1" in diameter. Sometimes I have to go beyond that, but then
I put a facing on the fence, which eases just about all the problems.
Charlie Self
"Half of the American people have never read a newspaper. Half never voted for
President. One hopes it is the same half." Gore Vidal

If you have the right saw to do that then it should be fine. What I mean by
the right saw is: Does the saw have an adapter to take the 5/8" hole in the
10" blade, some older saw allowed you to change arbors. 12" saw come stock
with a 1" arbor.

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